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Weigh power, heat factors in new resistors

Posted: 17 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:metal-film resistors? power and heat factors? better heat dissipation?

While resistors come in many flavors, the two most commonly used for power applications are metal film and wirewound. The biggest challenge for resistor manufacturers is packing more power into the same package size, whether it's for use in small electronic devices or large industrial equipment. And while a higher power rating is desirable, it can cause an undesirable amount of heat generation.

Many engineers will still swear by outdated carbon compound resistorsdespite availability issues and high price tagsfor specific power applications because they are inductance-free. But for this discussion, the focus is on metal-film and wirewound resistors.

In the resistor world, the bigger the package, the more power the resistor can handle. But today's market increasingly demands smaller electronic products, which translates into a need for smaller components. Add to this mix requirements for better heat dissipation and tighter tolerancewhich go hand-in-hand with more powerand you've got a big challenge.

Beefed-up power lines
Resistor manufacturers are beefing up their power lines to offer more power and better heat dissipation in same-size or smaller packages. In addition, several offer resistors that can operate at 275C to withstand harsh environmental conditions, including those found in automotive and industrial applications. Others have developed heat-sink resistors and surface-mount packaging for these devices.

"The downsizing of standard electronic devices is forcing higher power to be dissipated in a smaller board space, so you need power resistors that can handle the higher power per square inch," said Kory Schroeder, product engineering manager for Stackpole Electronics.

Power rating vs. heat rise is also critical for characterization, Schroeder said. The key concern with power resistors is meeting the upper limit of 105C for PCB temperatures. Designers start to get nervous if the board heat approaches that temperature limit, he said.

In addition, more-efficient power supplies mean that resistors need to have tighter tolerances and better temperature coefficients of resistance. Before the miniaturization trend, tolerance wasn't critical, but designers now regularly request 5 percent, Schroeder said.

On the flip side, industrial equipment is getting bigger and offering greater functionality, a trend that requires higher-power resistors, Schroeder said.

In general, the decision to use a metal-film or wirewound power resistor is determined by the power rating. If the requirement is below 5W, the typical choice is a metal-film resistor; above 5W, designers go with wirewound. That means that overlap applications are typically found at 5W.

KOASpeer's WK73 flat-chip resistor has better thermal-shock characteristics and high solder-joint solderability, thanks to its wide-terminal construction.

Product upgrades
These trends have led to new product developments over the past year.

Manufacturers such as Stackpole and BI Technologies have addressed the need for better heat dissipation. Stackpole retooled its HPC series with surface-mount devices in a 0.5inch? footprint capable of 5W of continuous power in free air. The parts use the same board space as typical 3W wirewound surface-mount devices. In addition, with 200lfm or 400lfm air movement, the power ratings improve to 10W and 12.5W, respectively, without exceeding a circuit board temperature of 105C, Stackpole said.

Stackpole also introduced its RHC high-power, thick-film chip resistors in a 2512 package that can handle 2W. Designed with specialized materials and processes, the chip resistors run 30C cooler than standard 2512 chips and exhibit solder-joint temperatures well below 105C.

Stackpole also added 30W and 50W offerings to its TR series of TO-220 power-film resistors. This type of resistor is typically mounted along with other power ICs to a heat sink maintained at 25C, which allows these small parts to dissipate large amounts of power.

Vishay offers the D2TO20 20W thick-film power resistor in an easy-to-mount TO-263 (D2PAK) package with resistance values from 0.01 to 500k.

Meanwhile, the BHP75 series of 75W power resistors from BI Technologies uses three heat-dissipation methods. The series is housed in a TO-220 open-screened substrate package and features an insulated tapered venturi that's bonded to the substrate for maximum heat dissipation. The design forces hot air up the venturi and away from the resistor.

Still, more power is the name of the game.

KOA Speer Electronics Inc. has released the WK73 wide-terminal flat chip resistor, which offers higher power ratings and enhanced heat dissipation. The WK73S3A in a 1225 case size offers a 1.5W rating, compared with the company's RK73B3A in a 2512 case size with a 1W rating.

For accuracy and stability, MHP TO-247 power resistors from TT electronics plc's IRC Advanced Film Division are available in 100W and 140W packages. They exhibit very low inductance and low thermal resistance.

Stackpole now has 20-, 25- and 30W power film resistors in a surface-mount package. The company also added 100W and 250W offerings to its KAL aluminum-housed wirewound power resistors.

Resistors with the ability to withstand higher temperatures include offerings from TT electronics' IRC Wire and Film Technologies Division. The CCW axial wirewound, CAW wirewound and CAF film power resistors operate at up to 275C. The IRC CMO metal oxide resistors, in standard and miniature sizes with power ratings from 0.25W to 9W at 70C, offer a maximum operating temperature of 240C.

Among the vendors achieving higher resistance and voltage ratings is Vishay Intertechnology Inc., whose RoHS-compliant D2TO20 20W thick-film power resistor has a resistance range of 0.01 to 500k in a compact, TO-263 package.

- Gina Roos

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